There is a need in the web press industry to control the length of the final, untensioned product. This is especially important when working with thin, low-stiffness substrates that are used, for example, in the packaging industry. In general, thin, low-stiffness substrates have a thickness of from about 0.0003 to about 0.0030 inches, and a tensile stiffness below about 500 lbs. per inch. Nonlimiting examples of such materials are PET film (0.00048 inches, 360 lb./inch), EUR70 film (0.0026 inches, 423 lb./inch), and BOPP film (0.0009 inches, 223 lb./inch).
In order to control the untensioned product length it must first be measured. In today's industry, the untensioned product length is sometimes measured directly. This can be accomplished by cutting a sample from the web, laying it out flat and untensioned on a table, and measuring it with a mechanical or video measurement system.
Attempts have also been made to define the untensioned product length by estimating the amount that a given substrate will “snap back” when process tensions are removed. One example of this appears to be the Muller Martini “Stretch Correct” system, as described in a technical article entitled “Web-offset to gain new fields of application: StretchCorrect—a striking innovation for film printing”, in NarroWebTech 4-2009, pages 12-14 (November 2009).
TecScan's Web Ranger system claims to “display in real time print length measurements of every repeat”. TecScan Web Ranger Brochure (2005). Assuming arguendo that this system works as described, it is believed to at most be able to approximate the repeat length of a strained (tensioned) web only.